To employ trim effectively, it’s best to follow a few basic precepts:

Use it consistently.
Your home will look best if all the trim work conforms to a single architectural style. Formal rooms, such as living rooms, may have more trim—wall paneling, a chair rail, or fancy cornices—than bedrooms, but all the trim should be of a consistent style.

Throughout your house, be careful not to go overboard with trim. Too much can overpower a room and detract from all other interior elements, decreasing rather than adding to your home’s attractiveness and value.

Make sure all the elements of your trim go together.
Within any style of trim, there are a number of choices. Particularly where trim pieces intersect, your selections need to be suited to one another. For instance, baseboard and chair rail should never be thicker than door and window casings.

The vertical pieces on a paneled wall should never be thicker than the horizontal trim they abut. In cases such as where a plate rail meets a window casing, the end of the rail needs to be finished with a return so the end grain is covered.

Develop your craftsmanship.
Woodworking skills are important when it comes to installing trim. Narrow gaps are almost imperceptible when filled and sanded, but wide gaps and uneven cuts offend the eye. This is especially true with wood that will be stained rather than painted. If you are going to install the trim yourself, experiment with cuts and joints on scrap wood before working with the real thing.

Keep it to scale.
The size of your trim and mouldings should be in proportion to the size of your rooms. Unfortunately, there are no formulas to help you determine what’s right for any specific situation.

As a rule of thumb, the baseboard and crown moulding should be in balance so that one doesn’t overpower the other. In rooms with a standard 8-foot-high ceiling, no baseboard or crown should be more than 6 inches tall. Most trim between 3 1/2 and 6 inches in height works well in these rooms.

Higher ceilings demand taller and deeper trim. In rooms where ceilings are extremely high, 6 inches is the minimum height for baseboard and crown moulding. If the doors in such a room are standard size, you can make them seem larger by installing wide casings or adding decorative treatments such as a pediment above or pilasters alongside them.

Establishing scale and proportion is a challenge, especially in a unique situation. If you don’t trust your own eye to guide you, you might want to hire a designer to specify what you should use and where you should place it before you invest in the material.